In the related art, there is a known image processing device in which a fluorescence image and a reflected-light image acquired from the same subject are combined to be displayed on the same screen by assigning different display colors thereto (for example, see Patent Literature 1). In other words, depending on the difference in brightness between the fluorescence image and the reflected-light image, a region in which the fluorescence has a relatively greater brightness is displayed with a different color from a region in which the reflected light has a relatively greater brightness, and thus, a fluorescent region with a greater brightness is displayed conspicuously.